1. Field of the Invention
This invention relates to the use of-a magnetic field as a means of applying a time-varying force to a force-sensing cantilever in an atomic force microscope in order to enhance the imaging sensitivity of the microscope through a scheme of synchronous detection of the corresponding modulation in the cantilever deflection signal. The cantilever may be coated with a magnetic material in which case the force may be applied either as a result of the torque induced by a magnetic field on a permanently magnetized cantilever or as the result of the force on a permanent or induced magnetic moment that arises from gradients in the applied field.
2. The Prior Art
The atomic force microscope works by scanning a force-sensing probe over a surface and in close proximity to it. Small deflections of the probe are sensed and used to maintain a constant interaction force (i.e., constant height of the probe above the surface). By recording the adjustments needed to maintain the interactions constant, a topographical map of surface features may be made [Binnig, G., et al., Atomic Force Microscope, Physical Review Letters 56(9), pp. 930-933 (1986)].
An improvement results if the position of the force-sensing probe is modulated (at a frequency, f) and the corresponding ac signal detected. This is because (a) this can result in reduced bandwidth (and hence reduced noise) if synchronous (i.e., lock-in) detection is used and (b) the amplitude of the ac signal is a measure of the derivative of the interaction force versus distance. This quantity changes much more rapidly than the interaction force itself. It is positive in the region where interaction forces are attractive (usually at large distances) and negative where interaction forces are repulsive (at short distances, i.e., in contact). Such a scheme has been described for non-contact AFM operation in air by Martin et al. [Martin, Y., et al., Atomic force microscope-force mapping and profiling on a sub 100 angstrom scale, Journal of Applied Physics 61(10), pp. 4723-4729 (1987)].
It has generally been believed that such operation in fluid would not be possible because of hydrodynamic damping of the cantilever. However, two groups have recently demonstrated that sensitive ac detection of the force gradient is possible in fluids. In one case, this is done by modulating the position of the whole sample cell [Hansma, P. K., et ai., Tapping mode atomic force microscopy in liquids, Applied Physics Letters 64, pp. 1738-1740 (1994)]. This is illustrated in FIG. 1. A laser beam 10 is reflected off a force-sensing cantilever 12 deflections of which are detected by a position sensitive detector 14. The cantilever is scanned over the sample surface 16 by a piezoelectric transducer 18. The force-sensing cantilever is immersed in a fluid body 20 contained in a fluid cell 22. The fluid cell 22 is mounted on a second transducer 24 which can displace the fluid cell 22 and sample surface 16 relative to the scanning transducer 18. An ac signal 26 is applied to the second transducer 24 so as to modulate the gap 28 between the force sensing probe tip 30 on cantilever 12 and the sample surface 16. The corresponding modulation of the laser beam position is detected by the detector 14. A synchronous detector 32 determines the amplitude and phase of the modulation. These signals are used to set the operating point of the microscope. For example, in the repulsive (contact) region of interaction between the atoms on the tip and the atoms on the sample surface, the tip deflection is in phase with the modulation and the amplitude decreased by closer contact with the surface. Thus, the height of the tip is adjusted so as to give a constant, in phase, reduction of the modulation signal as the tip is scanned over the surface. A plot of these adjustments as a function of the position of the tip in the plane of the surface constitutes a topographical height map of the surface taken at constant interaction-force-gradient.
A second group [Putnam, C. A. J., et al., Viscoelasticity of living cells allows high resolution imaging by tapping mode atomic force microscopy, Applied Physics Letters, submitted (1995)] have shown that similar results may be obtained by applying the modulation signal to the scanning transducer. This is illustrated in FIG. 2. The components are the same as those shown in FIG. 1, with the exception of the second transducer 24 which is omitted in this case. The gap 28 is modulated directly by a signal 26 applied to the scanning transducer 18.
Both of these approaches suffer from several drawbacks. The frequency of modulation is limited by the low-resonant frequency of the parts that are being displaced. In one case (FIG. 1) this is the whole sample cell. In the other case (FIG. 2) it is the whole scanning assembly. Furthermore, these complex assemblies have many resonances, not all of which cause the tip to be displaced with respect to the surface. Furthermore, these schemes make use of piezoelectric transducers which require high voltages for their operation, a requirement that imposes constraints when the microscope is operated in conducting liquids.
An alternative method of modulating the gap has been proposed by Lindsay in U.S. patent application Ser. No. 08/246,035 (incorporated herein by reference) and others [O'Shea, S. J., et al., Atomic force microscopy of local compliance at solid-liquid interfaces, Chemical Physics Letters, submitted (1995)]. In this approach, a magnetic particle or coating is applied to the force sensing cantilever and an external magnetic field is used to apply a force to the cantilever. This approach works in different ways, depending upon the direction of magnetization of the particle or film. O'Shea, et al. do not describe a method for controlling the direction of the magnetization of the film on the cantilever. Neither do they describe a method for producing force-gradient images by using this technique to modulate the gap between the tip and sample. Lindsay has described a reproducible method for applying a force to a cantilever using an external magnetic field and also a method for forming images by modulating the position of the cantilever with this technique. However, the magnetic arrangement described by Lindsay requires either a large magnetic moment on the tip or a large field gradient.
Two approaches to applying a magnetic force are illustrated in FIGS. 3 and 4. FIG. 3 shows the arrangement described in Lindsay. A magnetic particle or film 33 is magnetized so that the resulting moment, M (34) lies perpendicular to the soft axis of the cantilever 36. Then a magnetic field gradient 38 is applied in the same direction as M (34). The result is a force that is normal to the soft axis of the cantilever. A second arrangement is shown in FIG. 4. Here, a film, or particle 39 attached to the cantilever is magnetized with a moment M(40) along the soft axis of the cantilever 42. A magnetic field B (44) is applied normal to the soft axis of the cantilever. This results in a torque N=M.times.B. Thus, with a cantilever of length R, a force F of magnitude F=.vertline.N.vertline./R is applied to the end of the tip perpendicular to the soft axis of the cantilever. This latter method is to be preferred. This is because it is generally easier to obtain a high value of magnetic field than a high value of magnetic field gradient.